1. The Field of the Invention
This invention relates to an emulsion-emulsion breaker composition and to a method of breaking an emulsion or a dispersion of an emulsion by use of a chemical breaker, which is incorporated into the emulsion and which allows a controllable, residue-free break of the emulsion. The invention is particularly useful for well treatment operations where emulsions or dispersions of emulsions are injected into subterranean formations. The method of this invention more particularly relates to incorporating within an emulsion a deactivated emulsion breaker which a protecting group temporarily protects. Preferably the protecting group is bonded to the emulsion breaker through an ester bond. The ester bond hydrolyzes to form the active breaker; the protecting group is removed. When there is a sufficient amount of active breaker in the emulsion, the emulsion will break into its separated phases.
2. Description of the Prior Art
Emulsions are widely used. They are especially useful in certain well treatment operations because they provide necessary rheological properties at significant fluid cost savings. For example, in hydraulic fracturing operations, use of a water-in-oil emulsion or a dispersion of a water-in-oil emulsion (an emulsion slurry) can reduce the oil cost while maintaining the desired viscosity properties. Nevertheless, breaking of the emulsion after injection is sometimes a beneficial, if not essential, step in the treatment. The rheological properties beneficial for fracturing are liabilities for cleaning the fracture. Easier cleaning would be possible if the break of the emulsion-fracturing fluid could be controlled.
Generally, emulsions may be broken by adding demulsifiers to the pre-existing emulsions. The demulsifiers act with the surfactants (which induce emulsification and encapsulation) to cause an inversion and separation of the emulsion phase. Unfortunately, adding demulsifiers to injected emulsions is impossible. When the fluids are not being pumped, mixing is limited to the interface. Pumping would require further displacement of the emulsion within the formation. Therefore, in well treatment operations, several alternative schemes are used. In one system, the emulsion surfactant is selected so that it will prefer to wet the surface of the formation rock. In this way, as the emulsion passes into the formation, the surfactant is removed from the emulsion in sufficient amount to cause separation. In a second system, a mixture of surfactants is selected so that the emulsion will become unstable above a certain temperature. As the fluid temperature rises toward the formation temperature, the emulsion breaks. In a third system, the emulsion may be broken mechanically. The emulsion droplets break when they are squeezed into pores within the formation. Improvements in this technology are still desirable.
This invention presents a novel emulsion-emulsion breaker composition and a novel breaker method wherein a breaker is initially protected, is incorporated within the emulsion, and subsequently loses its protection to form an active breaker which breaks the emulsion without producing undesirable residues in the formation. The invention allows greater control of the break with reduced fluid costs for the treatment fluid.